from man:
-L, --log
           Enable makepkg build logging. This will use the tee program to send
           output of the build() function to both the console and to a text
           file in the build directory named pkgname-pkgver-pkgrel-arch.log.
           As mentioned above, the build log will be localized so you may want
           to set your locale accordingly if sharing the log output with
           others.


in /etc/makepkg.conf

#-- Specify a directory for package building.
BUILDDIR=/tmp/makepkg


but the log is put in ${startdir} instead

